1. Field of the Invention
The present invention relates to a material having a denatured clay as a main constituent thereof, and more particularly, to a novel material having a structure in which the layering of inorganic layered compound particles is highly oriented, the novel material being flexible and having water resistance, gas barrier properties, and sufficient mechanical strength to be used as a self-supporting film. In the technical fields of packaging materials, sealing materials and insulating materials, there has been a strong demand for the development of materials that have high gas-blocking properties, high water-vapor blocking properties, that are water-resistant, pliable and have heat resistance so as to allow them to be used in wet environments. The present invention has been developed in the light of the above situations and provides a novel technology and a novel material having high water resistance, high pliability, excellent water-vapor barrier properties and gas barrier properties, and which can be suitably used in a gas barrier film or the like.
2. Description of the Related Art
Most gas barrier materials having excellent pliability have been manufactured hitherto using as a base an organic polymer material, although the gas barrier properties of such materials are arguably far from perfect. The heat resistance of such organic polymer materials is highest for engineering plastics, at about 350° C. For achieving gas barrier materials at temperatures higher than that there must be employed inorganic sheets or metal sheets. Inorganic sheets are obtained by working a natural mineral such as mica, vermiculite or the like, or a synthetic mineral, into a sheet shape. Such sheets, which have high heat resistance and which are used as temporary gas sealing members in gland packing, cannot however be molded compactly, which precludes blocking completely the paths in the sheet through which minute gas molecules flow. The gas barrier properties of inorganic sheets are thus not particularly high. Also, gaskets of consolidated graphite lack sufficient gas barrier properties, and the use temperature thereof is limited to about 450° C. When high gas barrier properties are required at high temperatures, therefore, it becomes necessary to use metal sheets. The use of metal sheets requires strong fastening mechanisms, while surface damage during fastening may give rise to leaks. Metal sheets are also problematic in that, for instance, they do not afford electric insulation and cannot adapt to volume changes of surrounding members during heating or cooling, as a result of which there may form gaps that give rise to leaks.
In applications that involve use under higher temperature conditions than ordinary temperature, for instance, gas sealing in chemical plants, there are required films capable of being used under higher temperature conditions than those of conventional materials. In terms of preventing health hazards, in particular, there are required heat-resistant and asbestos-free gas barrier materials for joint sheets. Also, microwavable and/or boiling-water heatable materials such as materials having high gas barrier properties, high water-vapor barrier properties, as well as hot water resistance at temperatures of 120° C. or above, are demanded in food packaging materials.
Inorganic layered compounds such as swelling clay or the like are known to form a film having evenly oriented particles by dispersing the inorganic layered compound in water or alcohol, spreading the dispersion onto a glass sheet and letting it stand to dry. For example, oriented specimens for X-ray diffraction have been prepared using this method (Haruo Shiramizu, “Clay Mineralogy (Nendo Kobutsu Gaku)—Basics of Clay Science”, Asakura Shoten, p. 57 (1988)). However, when a film was formed on a glass sheet, it was difficult to strip the inorganic layered compound thin film off the glass sheet, while cracks formed in the thin film during strip-off, among other problems that made it difficult to obtain a self-supporting film. Even if the film was stripped successfully off the glass sheet, the resulting film was brittle and lacked sufficient strength. To date, it has been difficult to manufacture an even-thickness film free of pinholes and having excellent gas barrier properties.
Meanwhile, various polymeric resins are used as molding materials, and also as dispersants, thickeners, binders, and as gas barrier materials having inorganic materials blended therein. For instance, a known film having gas barrier properties may be obtained by manufacturing a film having a thickness of 0.1 to 50 μm from a composition comprising 100 parts by weight of a mixture of (A) a highly hydrogen-bondable resin containing two or more carboxyl groups per molecule, such as polyacrylic acid or the like, and (B) a highly hydrogen-bondable resin containing two or more hydroxyl groups in its molecular chain, for instance starch or the like, to a weight ratio A/B=80/20 to 60/40, and 1 to 10 parts by weight of an inorganic layered compound such as a clay mineral or the like; and by subjecting then the film to a thermal treatment and an electron beam treatment (Japanese Patent Application Laid-open No. H10-231434). The above film is problematic, however, in that the main component thereof is a water-soluble polymer resin, so heat resistance is not very high.
Also, a laminated film having excellent moisture resistance and gas barrier properties, suitable for food packaging, can be obtained by laminating a layer composed of a resin composition comprising a resin and an inorganic layered compound between two polyolefin-based resin layers (Japanese Patent Application Laid-open No. H07-251489). In this case, however, the layer of resin composition comprising an inorganic layered compound is merely used as part of a multilayer film, and not on its own as a self-supporting film. Also, the heat resistance of such laminated films is governed by the organic material having the lowest heat resistance in the composition, in this case a polyolefin, which is a material that, ordinarily, does not afford high heat resistance.
Various clays such as smectite, mica, talc, vermiculite or the like are added as fillers to plastic with a view to enhancing the heat resistance and/or gas barrier properties of the plastic. Smectite, having high water dispersibility, is hydrophilic, and hence has low compatibility with hydrophobic plastic. It is thus difficult to achieve a high-dispersion composite of smectite as-is in plastic. When forming a composite with hydrophobic plastic, therefore, clay is reformed and is used as a denatured clay having controlled hydrophilicity/hydrophobicity (Masanobu Onikata, SMECTITE, Vol. 8, No. 2, pp 8-13 (1998)). There are two methods for manufacturing denatured clay. One method involves ionic exchange with quaternary ammonium cations or quaternary phosphonium cations. Depending on their type and the ratio at which they are introduced, these organic cations allow controlling hydrophilicity/hydrophobicity. It has been reported that, among such organic cations, using a quaternary phosphonium cation affords higher heat resistance than using a quaternary ammonium cation (Japanese Patent Application Laid-open No. H06-95290). Various solvents can be selected based on the hydrophilicity/hydrophobicity level. Another method for manufacturing clay involves silylation. Hydroxyl groups present at the ends of clay crystals react with an added silylating agent, as a result of which such ends can be made hydrophobic. In this case as well, hydrophilicity/hydrophobicity can be controlled based on the silylating agent type and the ratio at which it is introduced. These two reformation methods can be used in combination. However, no self-supporting film material has been developed thus far using such a denatured clay as a main component. Organification is carried out ordinarily using quaternary alkyl ammonium chloride reagents. This is problematic in that, as a result, high chlorine concentrations are generated even after washing with water, which disqualifies this approach for applications where chlorine contamination is undesirable.
Recently, there have been manufactured inorganic layered compound thin films using the Langmuir-Blodgett Method (for instance, Y. Umemura, Nendo Kagaku, Vol. 42, No. 4, 218-222 (2003)). This method, however, involves forming an inorganic layered compound thin film on a substrate surface finished with a material such as glass or the like, and precludes achieving an inorganic layered compound thin film strong enough for a self-supporting film. Various other methods have also been reported for preparing functional inorganic layered compound thin films and the like. For instance, there is disclosed a method for manufacturing a clay thin film in which an aqueous dispersion of a hydrotalcite-based interlayer compound is made into a thin film and dried (Japanese Patent Application Laid-open No. H06-95290); a method for manufacturing a clay mineral thin film in which the bond structure of a clay mineral is oriented and fixed through a thermal treatment that promotes a reaction between the clay mineral and phosphoric acid or phosphoric acid groups (Japanese Patent Application Laid-open No. H05-254824); and an aqueous composition for a coating treatment, containing a complex compound of a divalent or higher metal and a smectite-based clay mineral (Japanese Patent Application Laid-open No. 2002-30255), to cite just a few of many such examples.
However, none of the above methods affords an inorganic layered compound oriented self-supporting film having sufficient mechanical strength to be used as a self-supporting film, and being imparted with gas barrier properties according to highly oriented clay particle layers.
In the cosmetic and pharmaceuticals fields, meanwhile, there have been proposed composites of inorganic layered compounds and organic compounds, for example advantageous organic composite clay minerals (for instance, Japanese Patent Application Laid-open No. S63-64913 and Japanese Patent Publication No. H07-17371), or in the manufacture of a drug for treating wet athlete's foot, comprising a mixture of a clay mineral, an acid, and an enzyme (for instance, Japanese Patent Application Laid-open No. S52-15807 and Japanese Patent Publication No. S61-3767). Nevertheless, the fact remains that these organic composite clay minerals have failed thus far to be used as self-supporting films.
Meanwhile, fuel cells, which exploit the inverse reaction of water electrolysis, to generate electricity through a reaction between hydrogen fuel and oxygen from air, are being developed as a next-generation energy source. Herein, there is an urgent demand for solid-polymer fuel cells using hydrogen ion-conductive membranes that afford enhanced ion conductivity and durability at temperatures of about 100° C.
Although various conventional materials have been developed in the fields of packaging materials, sealing materials, display materials, fuel cell materials and the like, no film material has been developed to date that is pliable, highly heat-resistant, water-resistant, and hydrogen ion-conductive, and which has high gas barrier properties and high water-vapor barrier properties. It would be thus highly desirable to develop and to apply in practice, in the present technical field, a novel pliable and highly heat-resistant material in the form of a water-resistant film having sufficient mechanical strength to be used as a self-supporting film.